Cell migration is regulated by signals from cell-cell interactions, cell-matrix interactions and chemo-attractants and -repellants. Such signals are transduced and coordinated by cascades of post-translational modifications and protein-protein interactions. Specifically, Src family tyrosine kinases are activated by mitogens and adhesion receptors and phosphorylate proteins that organize the leading edge and focal adhesions in migrating cells. The Cullin 5 RING (CRL5) E3 ubiquitin ligase and SOCS proteins (phosphotyrosine-dependent substrate adaptors for CRL5) have been implicated as potential tumor suppressors. We have found that SOCS-CRL5 complexes regulate Src activity, cell migration, proliferation and/or transformation in several cell types. Inhibiting the expression of SOCS proteins or Cul5 in epithelial cells stimulates migration, proliferation, and Src kinase activity. Increased Src is necessary but not sufficient for the increased migration and proliferation of CRL5-deficient cells, suggesting that SOCS-CRL5 complexes have additional substrates. We propose three broad aims to understand in depth how CRL5 regulates Src and cell migration. First, we will determine how CRL5 regulates Src activity. Second, we will identify CRL5 substrates, focusing on those that contain phosphotyrosine, and test which substrates regulate cell migration and proliferation. Third, we will dissect the mechanisms by which SOCS proteins and substrates coordinate focal adhesion and leading edge dynamics during cell migration. Collectively, our research will show how protein-tyrosine phosphorylation and ubiquitylation cooperate to regulate cell biology.